196 Vesuvius. [April, 



Having thus seen the potency of water under pressure as the 

 motive agent, and having the assurance of all observers, that water 

 is always present, we may conclude beyond doubt " that the rise of 

 lava in a volcanic vent is occasioned by the expansion of volumes 

 of high-pressure steam, generated in the interior of a mass of 

 hquefied and heated mineral matter within or beneath the eruptive 

 orifice." • 



During great eruptions much forked hghtmng has been ob- 

 served, darting forth in frequent flashes from the rising column 

 of smoke and ashes. Mr. Scrope thinks that the intense mutual 

 friction in the air of the ejected sohd matter is sutHcient to gene- 

 rate it ; Professor Phillips, on the other hand, suggests that its 

 occurrence is due to the rapid evaporization taking place and the 

 vast quantity of steam liberated producing electric tension in a 

 high degree.* 



Professor Phillips mentions the observations of Professor PiUa, 

 of Pisa, as to the occurrence of fames dimng volcanic eruptions. 

 He evidently thinks them deserving of full credit, although he 

 admits that in the majority of cases the hght emanates from the 

 surface of incandescent lava. AVe doubt one part of Professor 

 PiUa's observation, that "Hydrogen was perceived hj the smell ! "f 

 Sui'ely this should read " SuljjJmreffed Hydrogen," 



The theories as to the source of volcanic heat are many and 

 various, but the one which may be said to claim the largest share 

 of support is that which attributes the phenomena of volcanoes and 

 earthquakes to the reaction of the interior of our planet upon its 

 uppermast strata. 



A volcano therefore exists only where there is a permanent 

 connection between the interior of the earth and the atmosphere. 



That such interior possesses a very high degree of heat is 

 shown, not only by the state of fusion in which lava issues from 

 volcanic vents, but also because, in proportion to the depth, there 

 is a gradual increment of temperature observable so far as the 

 solid crust has yet been pierced by artificial means. 



Mr. Hopkins nearly thu'ty years ago, and subsequently Arch- 

 deacon Pratt and Sir Wilham Thomson, have condemned ;;s un- 

 tenable, and contrary to the known laws of precession and nutation, 

 the notion of a globe with a moderately thick crust and a fluid 

 interior. 



In order to produce a complete accordance in the motion of the 

 entire mass, it is necessary, according to these authorities, to assign 

 a sohd crust of at least 800 to 1000 English miles in thickness. 



Can any one believe that lava is pressed up through channels of 

 that length ? 



* r. H4. t r. 1^)4. 



